Government regulations for automobile passenger restraint systems appear to be headed in the direction of requiring an inflatable bag that momentarily and temporarily restrains a passenger during the critical instant of a collision impact. For safe and successful use the bag must be inflated in a very short time and thereafter deflated to release the passenger. The gas used to inflate the bag must be cooled enough to avoid damage to the bag and injury to the passenger. The gases used must also have a low toxicity.
Such a passenger restraint system must be installed in the automobile at the time it is built and may remain unused for a period of several years and still be ready for instant operation in case of a collision. The system should be economical since large numbers of installations are required and a significant economic impact results from relatively small changes in cost. Both cost and reliability considerations dictate a system with relatively low complexity. In addition, such a system must be stored behind the dashboard of the car until ready for use and a minimum volume is therefore essential.
There are three basic types of inflation systems for automobile passenger restraint bags. One of these has a sufficient supply of inflation gas stored under elevated pressure in a vessel that is opened at the moment of impact. In a collision such vessels may be opened explosively so that the pressurized gas is rapidly available. The adiabatic cooling of the gas as it expands from elevated pressure to the nearly ambient pressure of the inflatable bag introduces a severe chill and also reduces the effective volume of the gas available for inflating the bag.
Another technique employs a pyrotechnic gas generator wherein a rapidly burning composition produces substantial volumes of hot gas which are then directed into the inflatable bag. Some compositions are available with sufficiently low temperature of the combustion gas that it can be directed substantially directly into the bag without danger to the passenger. Other systems produce sufficiently high temperature gas that means must be used for cooling the gas before it is introduced in the bag. Various endothermic compositions, heat absorbing baffles, and the like have been devised for cooling the gas. However, these usually introduce substantial complexity into the system.
The third type of passenger restraint bag inflation system is a "hybrid" in that both a supply of gas under elevated pressure and a pyrotechnic gas generator are used. The gas from these two sources is mixed prior to inflating the bag and the high temperature of the combustion gas offsets the adiabatic cooling of stored gas to enhance gas volume and provide inflation gas at any desired temperature. In some such systems the pyrotechnic gas generator produces a rather small volume of gas but provides considerable energy for warming the gas from the high pressure gas storage vessel. This invention concerns a hybrid gas generation system.
Hybrid systems have an inherent complexity due to having two separate gas supplies. They also have tended to have a rather large volume and complex geometry in attempts to satisfy the safety and reliability requirements of the two systems. This has made it difficult to adapt the hybrid systems to practical installation in automobiles. One problem with hybrid gas generation systems has been that the pyrotechnic gas generator is often placed within the gas pressure reservoir so that the gas is mixed before leaving the reservoir. Pyrotechnic gas generators are initiated by an electrical signal which must be conveyed into the gas storage reservoir. Since the reservoir must hold the gas for a long period of time with high reliability, there are significant problems with long term sealing around the connectors leading the electrical wires into the reservoir. Such an arrangement also requires that the pyrotechnic gas generator be in place before the reservoir is filled and before it is sealed. The presence of the pyrotechnic material within the gas reservoir can limit the available techniques for sealing the reservoir thereafter. If the gas generator is external to the storage reservoir plenums are needed for mixing the gases before discharge into the inflatable bag. Such additional "plumbing" complicates the design and makes a bulky system that is difficult to fit into a car.
It is therefore desirable to have a hybrid gas system for an automobile passenger restraint bag that has a minimum volume and "clean" shape so that it can be readily fitted behind the dash of an automobile without extensive modification. Such a system should be reliable and economical and sufficiently sturdy to withstand the temperatures and pressures involved in operation.